1. Technical Field of the Invention
The present invention generally relates to clock gater circuits. More particularly, and not by way of any limitation, the present invention is directed to a clock gater having a reduced set-up time for a qualifier signal thereof with no area impact.
2. Description of Related Art
Many, if not most, of the integrated circuits (xe2x80x9cICsxe2x80x9d) currently being produced include some number of clocked logic circuits. A clocked logic circuit is one that performs its function subsequent to the assertion (or deassertion) of a clock signal. For example, some logic circuits (e.g., static logic circuits) cannot perform their intended functions until a clocked enable signal is asserted. Other logic gates (e.g., certain dynamic logic circuits) are alternately precharged and enabled by a clock signal and can only perform their intended functions during the enable phase of the clock signal.
Large-scale microprocessor ICs are designed to include clock distribution systems for distributing clock signals to various circuits on the IC and implement an overall clocking strategy. Typically, such clock distribution systems include devices called xe2x80x9cclock gaters,xe2x80x9d or simply xe2x80x9cgatersxe2x80x9d. Gaters buffer the clock signal, restoring strength thereto so that it is capable of driving a circuit.
It will be recognized that, in some instances (e.g., in order to conserve power or reduce heat generation), it will be necessary to inhibit operation of one or more circuits on the IC chip for a predetermined number of clock cycles while the remainder of the chip continues to function. In this regard, clock gaters can function as a simple on/off switch for the clock, controlling whether or not the clock signal flows through to the clocked circuit, thereby controlling whether or not the circuit operates.
FIG. 1A is a schematic diagram of a conventional prior art clock gater circuit 100. The gater circuit 100 includes an independently-controlled push-pull inverter 102 to create an output clock signal (xe2x80x9cCKxe2x80x9d) from an input clock signal (xe2x80x9cSLCBOxe2x80x9d). In particular, an push control signal NPU is connected to a PFET 104a of the inverter 102; this comprises the xe2x80x9cpushxe2x80x9d structure. Similarly, a pull control signal PD is connected to an nFET 104b of the inverter 102; this comprises the xe2x80x9cpullxe2x80x9d structure. In normal operation, a signal output from the inverter 102 flips up or down depending on the state of the push and pull control signals.
A qualifier (xe2x80x9cQUALHxe2x80x9d) signal, latched by a gate 106, can enable or disable the pull control signal PD on the push-pull inverter 102, thus respectively enabling or disabling the flow of the clock signal through the circuit 100. The primary problem with the clock gater circuit 100 is that the set-up time, that is, the amount of time prior to the firing of the input clock the QUALH signal needs to be asserted or deasserted, is significant. As a result, if the QUALH signal does not transition sufficiently early relative to the input clock signal, the output clock signal CK will not be enabled or disabled as desired. In the case of failure to disable the clock, this can result unnecessary power consumption and heat generation. Additionally, if the qualifier signal is too late or too early (i.e., does not meet timing requirements), the resulting extra or nonexistent clock could cause the processor to perform a calculation incorrectly. FIG. 1B is a gate level block diagram of the circuit 100. As best shown in FIG. 1B, the input clock signal SLCBO is delayed by a gate 120 to form a delayed clock signal ckd, which in turn is NANDed with the QUALH signal by a NAND gate 122 to produce an FBN signal. The FBN signal is NORed with the input clock signal SLCBO by a NOR gate 124 to produce the pull down control signal PD, which is applied to the gate of the FET 104b. The output clock signal is fed back and NANDed with the input clock signal SLCBO by a NAND gate 126 to produce the push up control signal NPU, which is applied to the gate of the FET 104a. The drains of the FETs 104a, 104b, designated as an nck node 127, are connected to the input of an inverter 128, the output of which comprises the output clock signal CK. The output of the inverter 128 is connected to the input of a second inverter 130, the output of which is connected to the input of the inverter 128 to form a feedback loop.
Operation of the gater circuit 100 is illustrated in a timing diagram in FIG. 2. As illustrated in FIG. 2, a waveform 200 represents the voltage level (from 0.0 V to 1.2V) of the input clock signal SLCBO over time (in nanoseconds (xe2x80x9cnsxe2x80x9d). Two and a half cycles of the input clock SLCBO comprising three rising edges 201a, 201b, and 201c, and two falling edges 202a, 202b, are illustrated in FIG. 2. Waveforms 204a and 204b represent QUALH signals asserted and subsequently deasserted at different points in time with respect to the input clock signal SLCBO. In particular, the QUALH signal represented by the waveform 204a is asserted and subsequently deasserted approximately 10 ps earlier in time than the QUALH signal represented by the waveform 204b. 
As will be noted with reference to FIG. 2, the earlier QUALH signal, represented by waveform 204a, is asserted (as represented by a rising edge 205a thereof) a time t1 after the immediately preceding falling edge 202a of the input clock signal SLCBO (waveform 200). Because the length of time t1 is short enough to meet the set-up time requirements, the next cycle of the output clock signal CK is enabled, as represented by a waveform 208a. In contrast, the later QUALH signal, represented by waveform 204b, is asserted (as represented by a rising edge 205b thereof) a time t2 after the immediately preceding falling edge 202b of the input clock signal SLCBO (waveform 200). In this case, because the length of time t2 does not meet set-up time requirements, the next cycle of output clock signal CK is suppressed, as represented by a waveform 208b. 
Similarly, the first QUALH signal (waveform 204a) is deasserted (as represented by a falling edge 210a thereof) a time t3 after the immediately preceding falling edge 202b of the input clock signal SLCBO (waveform 200). Because the length of time t3 meets the set-up time requirements, the deassertion of the QUALH signal results in suppression of the next cycle of the output clock signal CK, as represented by a waveform 212a. In contrast, the second QUALH signal (waveform 204b) is deasserted (as represented by a falling edge 210b thereof) a time t4 after the immediately preceding falling edge 202b of the input clock signal SLCBO (waveform 200). Because the length of time t4 does not meet the set-up time requirements, the deassertion of the QUALH signal results fails to suppress the next cycle of the output clock signal CK, as represented by a waveform 212b. 
FIG. 2 illustrates the concept of negative set-up time. In particular, a positive set-up time indicates a point in time prior to firing of the input clock signal within which the QUALH signal must be asserted/deasserted in order to respectively enable/disable the output clock signal. A negative set-up time indicates a point in time following firing of the input clock signal before which the QUALH signal must be asserted/deasserted in order to respectively enable/disable the output clock signal for the immediately succeeding cycle. In the examples shown in FIG. 2, the set-up time is approximately xe2x88x9270 picoseconds (xe2x80x9cpsxe2x80x9d), meaning that the QUALH signal must be asserted/deasserted before 70 ps after the firing of the input clock signal in order to effectively enable/disable the output clock signal for the immediately succeeding cycle. As shown in FIG. 2, times t1 and t3 meet the set-up time requirements (i.e., are less than or equal to 70 ps); times t2 and t4 do not (i.e., are greater than 70 ps).
FIG. 3 illustrates a timing diagram that is identical to the timing diagram illustrated in FIG. 2 except that it includes several additional waveforms 300a, 300b, 302a and 302b. The waveforms 300a and 300b represent the voltage level of the control signal PD respectively corresponding to the earlier-asserted/deasserted QUALH signal (waveform 204a) and the later-asserted/deasserted QUALH signal (waveform 204b). The waveforms 302a and 302b represent the voltage level of the nck node 127 respectively corresponding to the earlier-asserted/deasserted QUALH signal (waveform 204a) and the later-asserted/deasserted QUALH signal (waveform 204b).
At least one attempt has been made to design a xe2x80x9cfast qualifierxe2x80x9d; that is, a gater with a lower set-up time than conventional gaters such as the one illustrated in FIGS. 1A, 1B, and 2. This attempt, resulted in a gater that achieved a 90 ps set-up time advantage over a conventional gater on the falling edge of the qualifier signal in a 0.18 xcexcm process; however, the rising edge of the qualifier signal could push the output clock edge out over 100 ps, depending on the arrival time of the qualifier signal with respect to the input clock. Additionally, this design had an interstitial precharger in the pull-down structure, which reduced gain, and retained all of the original FETs, requiring a second qualifier for proper operation. This design also resulted in a net increase of two FETs over a conventional gater and therefore occupied more space on the IC chip than a conventional gater.
Accordingly, what is needed is a design for a clock gater for which the set-up time is not prohibitively large and that does not occupy more area on an IC chip than a conventional gater.
Accordingly, the present invention advantageously provides a system and method for implementing a clock gater circuit. One embodiment is a clock gater circuit comprising an output clock signal generator electrically connected between a clock input for receiving an input clock signal and a clock output; a switch for selectively enabling or disabling generation of a clock signal by the output clock signal generator; and circuitry for causing a voltage level of the clock signal generated by the output clock signal generator to maintain a current voltage thereof responsive to a qualifier signal voltage level provided thereto.